From an antenna perspective, there are some challenges especially when the nearest 3G base station may be in a different direction every day/night and where signal strengths and network availability may not be all they might. Therefore antennas that radiate in every direction (known as omni-directional antennas) are most appropriate.
So here is our take on three useful high-gain 3G/4G antenna options for waterways applications:
The first option is a high gain low profile magnetic mount antenna. It requires a reasonable-sized metal surface on which to mount, but supplied with typically 4M of cable which can be routed below deck, it can be flexible enough to provide a significant performance improvement over the antenna built-in to the dongle. With an optimum groundplane, peak gain before cable loss can be up to 4-5 dBi. An example of such antennas can be found here.
A low profile omni antenna is the second option. Whilst not offering as much gain as the magnetic antenna above, it can sit pretty much flush to the deck or roof of the boat. With a 2.5M cable and 3 dBi gain, the low profile can help "move" the signal to enable a dongle to be used inside the boat or below deck. The low profile antenna can be stud-mount or tape-mount. Examples can be found here and here.
The high gain omni-directional antenna option is a more traditional type antenna offering 4-5 dBi gain across the bands. A neat technique is to mount the bracket permanently somewhere on the boat (as high up as possible) and the antenna can be fitted and removed in a matter of seconds so if you are going to have height clearance issues on your travels then this will not cause to much hassle. To go with a high gain omni you would typically need some low loss cable to reduce losses down the cable run, but cables could be run up to 10M if required. An example of the antenna can be found here. A 5M compatible jumper cable looks like this.
In conjunction with these antennas, the correct connector for your 3G device is required. Typically, the following dongle and modem/router manufacturers lean towards the following connectors, though it is worth verifying before you purchase:
Huawei - Common connector CRC9 - CRC Adapters can be found here.
Sierra Wireless - Common connector TS9 - TS9 Adapters can be found here.
Option - Common connector MC-Card - MC-Card Adpaters can be found here.
Often 3G routers will have a SMA-Female Jack for the antenna port. A SMA-Female Jack looks like this:
That means that the antenna will require a SMA-Male connector. The SMA-Male plug looks like this: